The present invention relates to a vehicle-body lower structure of a vehicle which comprises a tunnel portion provided at a central portion, in a vehicle width direction, of a floor panel forming a bottom face of a vehicle compartment and a cross member provided between the tunnel portion and a connection portion of a center pillar to a side sill provided at a side portion of the vehicle compartment to extend in the vehicle width direction.
Conventionally, as disclosed in Japanese Patent Laid-Open Publication No. 2004-74835, in the floor cross member structure of a vehicle in which the floor cross member connects the side sill, to which the lower end portion of the center pillar is connected, and the tunnel portion formed at the floor panel and extends in the vehicle width direction, the hole is formed at the ridgeline portion of the terminal portion of the cross member connected to the side sill (rocker) which is located on the outside in the vehicle width direction so that the sectional resistance force of the connection portion of the cross member to the side sill can be set to be smaller than that of the cross member body.
Further, as disclosed in Japanese Patent Laid-Open Publication No. 2010-228482, aiming at proving the vehicle-body lower structure of a vehicle which can effectively absorb compressive and bending forces, restraining the buckling deformation of the cross member, and properly restrain the upper portion of the center pillar from coming into the vehicle compartment by ensuring the proper deformation of the lower portion of the center pillar, the side-sill-side end portion of the cross member is located at a position away from the side sill with a specified distance, the connection member which connects the side-sill-side end portion of the cross member and the side face of the side sill is provided, the connection member is configured to have a lower rigidity than the cross member, and the strength of the cross-member connection area of the connection member is configured to be higher than that of the side-sill connection area of the connection member.
In case the sectional resistance force of the connection portion of the cross member to the side sill is set to be smaller than that of the cross member body as disclosed in the above-described first patent document, deformation is made occur in the vicinity of the connection portion of the floor cross member to the rocker inner portion right after the vehicle collision happens, so that the lower portion of the pillar can be made come into the vehicle compartment to restrain the upper portion of the pillar from falling in. However, in the late stage of the vehicle collision, it cannot be avoided that the upper portion of the center pillar falls into the vehicle compartment in a state in which the lower end portion of the center pillar is supported by the side sill and the cross member. Thereby, there is a problem in that the rotational moment acts on the side sill and the bending force of this rotational moment causes the bucking deformation to the cross member, so that the impact load from the vehicle side collision may not be properly supported by the cross member.
Further, since in the vehicle-body lower structure of a vehicle disclosed in the above-described second patent document, the impact load is received at the side-sill connection area with the low strength in the initial stage of impact-load input via the center pillar and the side sill and then the bending moment from the side sill is received at the cross-member connection area with the high strength, the bucking deformation of the cross member in the initial stage of vehicle side can be prevented. However, there is a problem in that the stress concentration occurs at the connection flange piece of the connection member to the cross member when the bending moment acts on the cross-member connection area and thereby this occurrence causes separation of these members, so that the impact load inputted to vehicle-body lower portion in the vehicle side collision may not be supported effectively.